This invention relates to a jet drop printing process and printing inks therefor and more particularly to radiation curable printing inks suitable for use in jet drop printers of the type disclosed in Lyon et al. U.S. Pat. No. 3,739,393; Brady et al., U.S. Pat. No. 3,805,273; Stoneburner U.S. Pat. No. 3,891,121; and Stoneburner U.S. Pat. No. 4,080,608. Such printers print on a high-speed moving web by selective charging and catching of drops generated at a frequency in the order of about 50 Khz by each of the streams flowing from 500 or more orifices. In the case of high speed printing, discrete droplets of ink must be formed at the rate of between ten thousand and one million droplets per second, each possessing essentially the same color characteristics for a given color. Usually, the drops range in size from 2 to 3 mil droplets to give deposited droplets of 3 to 5 mils in diameter, when generated through an orifice of less than about 2 mils in diameter.
As can be imagined, the inks which find utility in such jet drop printers must have a number of special characteristics. Typically, water-based inks have been used because of their conductivity and viscosity range. Thus, for use in a jet drop printer the ink must be electrically conductive, having a resistivity below about 5000 ohm-cm and preferably below about 500 ohm-cm. For good runability through small orifices water-based inks generally have a viscosity in the range between about 1 to 15 centipoise at 25.degree. C.
Over and above this, the ink must be stable over a long period of time, compatible with the materials comprising the orifice plate and ink manifold, free of living organisms, and functional after printing. The required functional characteristics after printing are: sufficient light absorptivity at infrared wave lengths, smear resistance after printing, fast drying on paper, and waterproof when dry.
Examples of different types of water-based jet drop printing inks are found in U.S. Pat. Nos. 3,903,034; 3,889,269; 3,870,528; 3,846,141; 3,776,642; and 3,705,043. Several of these patents address individual problems inherent in use of water-based inks in jet drop printers. Thus, Sanders U.S. Pat. No. 3,776,742 utilizes a special formulation to provide an non-cockling feature and Ostergren U.S. Pat. No. 3,846,141 discloses use of a hermectant to prevent clogging of the nozzles through drying. Even, then, it can be said that no water-based ink has been developed which has fully satisfactory smudge resistance, drying and waterproof characteristics.
Thus, water-based inks in general can be said to have the following disadvantages:
(1) They require water evaporation in the drying stage, necessitating extensive drying systems which utilizes large amounts of energy
(2) Large printed areas of paper substrates usually cockle because of the amount of water present.
(3) The dried images are sensitive to wet and dry rub.
(4) The compositions usually have to be formulated to include an anti-baterial perservative when extended periods of storage are anticipated in order to prevent or minimize the growth of bacteria in the compositions.
(5) Inks of low viscosity have a tendency to tip dry on the orifice without the use of humectants, but the addition of humectants to prevent tip drying usually results in an increase in viscosity.
Of course, it is possible to solve some of the problems, such as absorbency and waterproof characteristics, by use of polar organic solvent-based inks. U.S. Pat. Nos. 3,994,736 to Hertz and 4,070,322 to Hwang disclose the use of polar organic solvent-based inks; and Nos. 4,021,252 to Banczak and 4,024,096 to Eztchel disclose use of alcohol-water solvent systems.
Still, many of the problems remain. In addition, use of organic solvents requires that precautions be taken against any toxic or flammable vapors which may be emitted. When the solvent content is high, this can be a particular problem in the vicinity of the drop generating orifice or orifices, and even removal of the volatile materials from the printed surface requires the use of bulky, elaborate drying systems. Because these problems are so serious, most prefer using water-based inks despite the number of disadvantages listed above. Accordingly, the need exists for a jet printing ink which has as few of these disadvantages as possible.
That need is met by the use of radiation curable inks of the type described herein. Of course, radiation-curable inks per se are known in the art. See, for example, U.S. Pat. Nos. 4,056,453; 4,026,949; 3,804,736; and 3,803,109. But, none of these mention using radiation-curable inks in jet drop printers. To my knowledge, that has not previously been suggested. In addition, the unique ink formulations of the present invention are particularly suited to use in jet drop printers.